1. Technical Field
The present invention relates to a liquid crystal display and a backlight unit for use in a liquid crystal display panel or the like and more particularly to a liquid crystal display and a backlight unit which increase a rate of maintenance of luminance and are effective in a reduction in deterioration of luminance.
2. Prior Art
Recently, a liquid crystal display has been widely adopted as a display device for an image display for a personal computer, a television or the like and for various types of monitors. In general, this type of liquid crystal display comprises a backlight placed at the back of a liquid crystal display panel, the backlight being a planar light source for lighting, wherein a liquid crystal surface having a predetermined spread is wholly illuminated with a light of uniform brightness, whereby an image formed on the liquid crystal surface is formed into a visible image.
In this backlight, a hot cathode or cold cathode fluorescent lamp is adopted as the light source. It is necessary to illuminate the whole surface of the liquid crystal display panel with the light from a so-called linear light source by such a fluorescent tube. Thus, two types, i.e., a directly-under-light type and a side-light type (an edge-light type) have been heretofore adopted. The directly-under-light type comprises the fluorescent tube placed directly under the liquid crystal display panel, and a dimmer and a diffuser disposed on the fluorescent tube. On the other hand, the side-light type comprises the fluorescent tube disposed along two sides or one side of a light guide made of a transparent resin, wherein the light entering into the light guide is directed toward the liquid crystal display panel surface by a reflector worked on the rear surface of the light guide, whereby a uniform and planar light is obtained by using light diffusion.
A hot cathode fluorescent tube is the same type as a general fluorescent tube. That is, the hot cathode fluorescent tube comprises a filament composed of a tungsten wire wound into a coil, the filament being coated with an emitter having a high electron emission coefficient. Therefore, the hot cathode fluorescent tube has a high current density and can thus reduce power consumption in electrodes. Accordingly, the hot cathode fluorescent tube is superior in high efficiency and high luminance. On the other hand, a cold cathode fluorescent tube has the same structure as the hot cathode fluorescent tube except for the electrodes. A mechanism for emitting electrons from the electrodes works not by the action of heat but by secondary electron emission. Although the cold cathode fluorescent tube has low efficiency and low luminance, the cold cathode fluorescent tube is therefore superior in long life.
To use these fluorescent tubes as the backlight for the liquid crystal display, the high luminance and high definition are required. Thus, a technique of disposing a plurality of fluorescent tubes adjacent and parallel to one another has been heretofore adopted. For example, the side-light type comprises the backlight composed of six fluorescent tubes in total, three fluorescent tubes being placed at each side of the backlight, each fluorescent tube having an outer diameter of about 3 mm. A plurality of fluorescent tubes are used as the backlight, whereby the liquid crystal display can be adopted not only into a portable personal computer and word processor but also into desktop information equipment and a television receiver which need the brightness. The CRT-level luminance can be therefore ensured even if transmittance of liquid crystal is taken into account.
The prior art using a plurality of fluorescent tubes as the backlight is disclosed in Japanese Patent Laid-Open Sho 62(1987)-234185, Hei 5(1993)-2165, Hei 10(1998)-177170 and so on. In these publications, it is borne in mind that the reflector is disposed in a housing for holding a plurality of fluorescent tubes and the reflector condenses the lights from a plurality of fluorescent tubes, thereby increasing the luminance. Furthermore, Japanese Patent Laid-Open Hei 10(1998)-177170 discloses the technique in which an electrostatic shielding component made of a conductive material such as aluminum is disposed between the cold cathode fluorescent tubes in order that a plurality of cold cathode fluorescent tubes glow with stability, thereby reducing an electrostatic capacity between the cold cathode fluorescent tubes.
A plurality of fluorescent tubes are thus used, whereby the luminance of a fluorescent tube unit comprising a plurality of fluorescent tubes can be increased. However, it became clear that the life of the fluorescent tube unit could not achieve a development goal. That is, although the lighting life of a single fluorescent tube can achieve a design goal, the deterioration of luminance of the whole fluorescent tube unit is considerable and thus the life cannot be ensured from the viewpoint of the deterioration of luminance.
FIG. 9 is a graph of a result of a continuous lighting life test, showing measured data of the side-light type backlight (an LCD module) using a plurality of cold cathode fluorescent tubes and measured data of a single cold cathode fluorescent tube (a single lamp). In the drawing, a vertical axis indicates a rate of maintenance of luminance and a horizontal axis indicates a lighting time. Besides a measured value of the single lamp, the measured values of four LCD modules, each LCD module using a plurality of cold cathode fluorescent tubes, are shown in the drawing. Generally, the life of the backlight is defined as the lighting time that elapses before the luminance drops to 50% of initial luminance. The greater deterioration of luminance means the shorter lighting time that elapses before the luminance drops to 50%. In the drawing, by comparison of the luminance after an elapse of the lighting time of about 2000 hours, it turned out that the luminance of the LCD module severely deteriorates to about 65% with few exceptions when the single lamp has the rate of maintenance of luminance of about 90%. That is, the extent of the deterioration of luminance is considerable in the case of the LCD module of the backlight using a plurality of fluorescent tubes. This becomes a major problem from the viewpoint of an insurance of the life.
The present invention is made in order to solve the above technical problem. It is an object of the present invention to reduce the deterioration of luminance that can occur in the backlight for the liquid crystal display panel.
As shown in FIG. 1, a liquid crystal display of the present invention comprises a light guide 2 placed at the back of a liquid crystal display panel 1; a first fluorescent tube 3 disposed along at least one side of the light guide 2; a second fluorescent tube 4 disposed adjacent to the first fluorescent tube 3; and a shielding component 5 for shielding an incoming light directly entering from the second fluorescent tube 4 into the first fluorescent tube 3.
The liquid crystal display further comprises a housing 6 having an inner surface 7 forming a reflecting surface, the housing 6 being opened to the light guide 2, the housing 6 being capable of disposing therein the first fluorescent tube 3 and the second fluorescent tube 4 substantially parallel to each other, wherein the shielding component 5 includes a shield 8 located between the first fluorescent tube 3 and the second fluorescent tube 4 and formed of a part of the housing 6 extending toward the light guide 2. The liquid crystal display is superior in that a rate of maintenance of luminance of a plurality of fluorescent tubes included in the housing 6 can be brought close to the rate of maintenance of luminance of a single fluorescent tube.
Preferably, the shield 8 extends to substantially the same position as a position d1 at which the first fluorescent tube 3 and the second fluorescent tube 4 are adjacent to the light guide 2. Thus, the incoming light directly entering from the adjacent fluorescent tube can be shielded.
Furthermore, in order to sufficiently shield the incoming light directly entering from the adjacent fluorescent tube, it is preferable that the shield 8 extends to a position d2 closer to the light guide 2 than the position at which the first fluorescent tube 3 and the second fluorescent tube 4 face the light guide 2.
The surface of the shield 8 forms the reflecting surface. Thus, it is possible to prevent the light from entering into the adjacent fluorescent tube and to sufficiently illuminate the light guide 2 with the lights emitted from the individual fluorescent tubes.
Although the first fluorescent tube 3 and the second fluorescent tube 4 are used in the present invention, the present invention can be similarly applied to third and fourth fluorescent tubes. Furthermore, even in the case of the single fluorescent tube made of a curved glass tube having a U shape or the like for example, assuming that one tube wall of the U shape or the like of the fluorescent tube is the first fluorescent tube 3 and the other tube wall of the U shape or the like is the second fluorescent tube 4, the present invention can also be grasped. Accordingly, the present invention is not limited to only the completely separate fluorescent tube.
Moreover, the shield 8 does not necessarily intend to completely shield even the indirectly incoming light resulting from diffused reflection or the like. Thus, the shield 8 has only to shield the directly incoming light from the adjacent fluorescent tube.
Furthermore, any type of fluorescent tube such as a hot cathode fluorescent tube or a cold cathode fluorescent tube may be used as the fluorescent tube. However, the present invention is applied to the fluorescent tube in which an electrode substance is sputtered and deposited on a lamp tube and thus a deterioration of luminance occurs, whereby the deterioration of luminance can be reduced. Accordingly, it is preferable that the present invention is applied to such a fluorescent tube.
Furthermore, the fluorescent tube has only to be disposed along at least one side of the light guide 2. However, the same function can be achieved even when the fluorescent tube is disposed along both of one side and another side.
Moreover, a liquid crystal display of the present invention comprises a light guide 2 placed at the back of a liquid crystal display panel 1; a plurality of cold cathode fluorescent tubes 3 and 4 disposed along at least one side of the light guide 2 and disposed substantially parallel to each other; and a separator 5 for separating the lights emitted from the plurality of cold cathode fluorescent tubes 3 and 4.
Preferably, the plurality of cold cathode fluorescent tubes 3 and 4 are held in a housing 6 disposed along one side of the light guide 2, and the separator 5 protrudes from the inner surface of the housing 6 toward the light guide 2. Thus, an influence of a reduction in transmittance of the glass tube constituting the cold cathode fluorescent tubes 3 and 4 can be reduced, and therefore the deterioration of luminance can be brought close to the deterioration of luminance of the single fluorescent tube. The cold cathode fluorescent tube, in particular, emits electrons by secondary electron emission. Thus, when an abnormal region is lighted at the time of discharge, the electrode substance is sputtered and the electrode substance is prone to be specularly deposited on the lamp tube. Therefore, the present invention is superior in that the influence of the reduction in the transmittance of the glass tube can be reduced even when such a cold cathode fluorescent tube is used.
Moreover, a liquid crystal display of the present invention comprises a diffuser and a dimmer or neutral density filter placed at the back of a liquid crystal display panel; a fluorescent tube placed at the back of the diffuser and the dimmer or neutral density filter; a housing for holding the fluorescent tube; and a shielding component disposed in the housing and disposed so as to reduce the number of times that the light emitted from the fluorescent tube passes through a tube wall of the fluorescent tube. Thus, the present invention is superior in that the deterioration of luminance can be reduced even when a so-called directly-under-light type backlight is adopted.
Preferably, a plurality of fluorescent tubes are included in the housing, and the shielding component is disposed between the plurality of fluorescent tubes and shields the directly incoming light from the adjacent fluorescent tube. Thus, the deterioration of luminance of a fluorescent tube unit comprising a plurality of fluorescent tubes can be brought close to the deterioration of luminance of the single fluorescent tube.
Furthermore, the fluorescent tube has a curved tube structure, and the shielding component is disposed between adjacent portions of the curved fluorescent tube. Thus, the liquid crystal display does not necessarily have a plurality of straight fluorescent tubes. Also when one or a few fluorescent tubes curved into the U shape or the like are used, it is possible to prevent a plurality of passages of the light through the glass tube constituting the curved fluorescent tube. As a result, the extent of the deterioration of luminance can be reduced.
The curved fluorescent tube may be U-shaped as described above or comprise a meandering glass tube. In the latter case, preferably, the shielding component is disposed between the adjacent portions of the glass tube.
Moreover, referring to FIG. 1, a backlight unit of the present invention comprises a housing 6 opened at one side, for accommodating a plurality of fluorescent tubes 3 and 4 therein; and a shielding component 5 disposed in the housing 6, for shielding the incoming light directly entering from the fluorescent tube 3 into the fluorescent tube 4 and vice verse.
The housing 6 is opened to the back or the side of a liquid crystal display panel 1, and the inner surface 7 of the housing 6 forms the reflecting surface. Thus, an insurance of life of a backlight can be appropriately provided regardless of the type of backlight such as the directly-under-light type or side-light type.
Furthermore, a backlight unit of the present invention comprises a fluorescent tube unit 9 composed of plural or curved fluorescent tubes 3, 4 and 10 placed at the back or the side of a liquid crystal display panel 1, wherein the fluorescent tube unit 9 reduces interaction by the incoming light among the fluorescent tubes constituting the fluorescent tube unit 9 so that a rate of maintenance of luminance of the whole fluorescent tube unit 9 may be brought close to the rate of maintenance of luminance of a single fluorescent tube 10 constituting the fluorescent tube unit 9.
Preferably, the fluorescent tube unit 9 reduces the number of times that the light in the fluorescent tube unit 9 passes through the tube walls of the fluorescent tubes 3, 4 and 10. Thus, the influence of contamination resulting from a deposition of a compound in the fluorescent tube can be minimized. Therefore, the rate of maintenance of luminance of the fluorescent tube unit 9 can be brought close to the rate of maintenance of luminance of the single fluorescent tube 10.
More specifically, the fluorescent tube unit 9 includes a separator 5 for separating the plural or curved fluorescent tubes 3, 4 and 10. Thus, such a function can be simply and fully achieved.
Furthermore, preferably, a liquid crystal display of the present invention comprises a liquid crystal display panel; and a fluorescent tube unit composed of plural or curved fluorescent tubes placed at the back or the side of the liquid crystal display panel, wherein at least one of the plural fluorescent tubes or the curved fluorescent tube constituting the fluorescent tube unit includes a reflecting film for reducing the interaction by the incoming light to or from the adjacent fluorescent tube. Without providing the housing 6 with the separator 5, the rate of maintenance of luminance of the fluorescent tube unit can be thus brought close to the rate of maintenance of luminance of the single fluorescent tube.
Specifically, the reflecting film is formed inside or outside a glass tube of the fluorescent tube. Furthermore, the reflecting film can have a predetermined aperture angle so as to shield the light at any angles other than the aperture angle. Furthermore, preferably, the aperture angle of the reflecting film is 77 degrees or less. Thus, it is possible to more effectively shield the incoming light to or from the adjacent fluorescent tube.
Moreover, a fluorescent tube for a backlight unit of the present invention, which is used in a state in which plural or curved fluorescent tubes are placed at the back or the side of a liquid crystal display panel, comprises a glass tube wall having a predetermined diameter; a fluorescent material provided on the glass tube wall and converting an ultraviolet light emitted from mercury excited by discharge into a visible light, thereby emitting the light; and a reflecting film formed on an inner wall or an outer wall of the glass tube and having a predetermined aperture angle in order to reduce the interaction by the incoming light to or from the adjacent fluorescent tube used for the backlight unit. When the fluorescent tube is used for the backlight unit, the deterioration of luminance of the unit including the adjacent fluorescent tube can be reduced.
Preferably, the aperture angle of the reflecting film is such an angle that the light does not reach to the adjacent fluorescent tube or such an angle that the light from the adjacent fluorescent tube does not reach into an aperture. Most preferably, the aperture angle is 77 degrees or less. Furthermore, a metalescent material such as aluminum or a white material such as titanium oxide is excellent for the material of the reflecting film.